In Ubuntu 11.04 (both classic desktop and Unity), you can trigger the Compiz Scale plugin using the SUPER + W keyboard shortcut but if you don't like using keyboard shortcuts, here's how to create a custom Unity launcher (but also works on as a Gnome Panel shortcut) that triggers the Compiz Scale plugin.

There's one limitation to this, because of the way the Unity Launcher works: when you click the Scale launcher, you can only click it one more time to exit scale and then you must wait a few seconds before being able to click it again. However, you can go around this limitation by middle clicking it.

Then open Nautilus, navigate to ~/.local/share/applications/ (.local is a hidden folder so press CTRL + H to see it) and you'll find a new "scale.desktop" file - drag this file to the Unity launcher. If you want to change the icon, open the scale.desktop file using Gedit and specify a different icon.